"The forensic review is ongoing," HBO said in an emailed statement, noting that it was working with cyber security experts and law enforcement.
The hack came at a sensitive time for HBO, whose parent Time Warner is awaiting regulatory approval to sell itself to AT&T in an $85.4 billion deal announced in October.
The Guardian and other publications reported on Tuesday that hackers had posted some stolen HBO files online, demanding a Bitcoin ransom to prevent additional releases.
Reuters was not able to view the leaked files or verify their authenticity.
Technology news site Mashable posted written demands that it said hackers claimed to have sent to HBO Chief Executive Officer Richard Plepler. The hackers said they had stolen a wide variety of confidential material including scripts to its programs, contracts, human resources documents and budget details.
A source familiar with the situation said the Federal Bureau of Investigation was probing the matter. An FBI spokesperson declined to comment.
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